Chart takes the size of its container at the time of rendering. In this case, probably kendo tabstrip hasn’t set its size when chart is being rendered and hence chart has rendered with default size. So, you can keep a reference of the chart and call chart.render() soon after the tab is opened and the chart gets resized according to its container’s size.
In case you are using our jQuery Charting plugin, you can get reference to chart by doing $(“#chartContainer”).CanvasJSChart(). You can learn more about jQuery plugin here.